Swing-type excavation machines, for example hydraulic excavators and front shovels, require significant hydraulic pressure and flow to transfer material from a dig location to a dump location. These machines direct the high-pressure fluid from an engine-driven pump through a swing motor to accelerate a loaded work tool at the start of each swing, and then restrict the flow of fluid exiting the motor at the end of each swing to slow and stop swinging of the work tool.
In order to improve the efficiency of this type of hydraulic arrangement, one or more accumulators are provided in fluid communication with the swing motor. Based on an operating state of the swing motor, the accumulators are charged or discharged. For example, the accumulators may be discharged to assist an acceleration of the swing motor. Further, the accumulators may be charged by fluid exiting the swing motor during a deceleration of the swing motor.
However, charging of the accumulators by fluid from the swing motor may reduce fluid pressure at an input port of the swing motor. This may result in cavitation, thereby damaging the swing motor and adjoining components.
U.S. Patent Publication 2011/0020146 discloses a variable displacement hydraulic pump supplying pressure oil to a hydraulic actuator, a pressure detector detecting a pump discharge pressure from the hydraulic pump, a control valve controlling a supply of the pressure oil to the hydraulic actuator, a controller controlling a pump displacement of the hydraulic pump, a hydraulic motor rotating an upper structure of the construction machine, a swing relief valve defining a relief pressure of the hydraulic motor, and a control lever switching a control valve for the hydraulic motor. The controller includes: an adjuster that, when a pump discharge pressure detected by the pressure detector exceeds a first set value, conducts an adjustment to reduce the pump displacement; and a canceller that cancels the adjustment when the pump discharge pressure falls below a second set value. The second set value is equal to or larger than the first set value.